CHNG II: CN BNG AXIT BAZ

I.CC AXIT- BAZ

I.1 nh ngha.

I.1.1 Theo Areniuit

- Axit l nhng cht khi ha tan vo H2O sinh ra H+

V d: HCl H+ + Cl-

- Baz l nhng cht khi ha tan vo H2O sinh ra OH-

V d: NaOH Na+ + OH-

I.1.2 Theo Bronsted.

- Axit l nhng cht c kh nng nhng proton.

- Baz l nhng cht c kh nng nhn proton.

Ty theo bn cht ca dung mi mt cht c th lm axit hc baz.

I.1.3 u im ca thuyt Bronsted

- C tnh khi qut hn cho php gii thch c tnh axit-baz trong cc dung

mi khc nhau mt cch nht qun

Vd: vi axit NH+4 gii thch theo Bronxtet

NH+4 + H2O NH3 + H3O+

Cn gii thch theo Areniut phi a ra khi nim thy phn

NH+4 + H2O NH4OH + H+

I.2 Cc phn ng ca axit baz trong nc.

I.2.1 Nc l cht in ly yu, lng tnh, va th hin tnh axit va th hin tnh

baz

H2O + p = H3O+

H2O p = OH-

2H2O = H3O + OH- k

k = 3 22

( ).( )( )

H O OHH O

2 0Hk = (H3O+).(OH-) Kw - Kw c gi l tch s ion ca nc. kw c gi tr 10-14 22oC

- Nu l nc nguyn cht [H+] = [OH-] = 10-7

- pH = - lg [H+]

pH < 7 => mi trng axit

pH > 7 mt baz

pH =7 mt trung tnh

V d

Thm 300ml H20 vo 200ml HCl 0,1M tnh pH sau phn ng

Gii: nng ban u ca HCl CHCl = = 0,04 (M)

[H+] = 0,04 => Ph = -lg(0,04) = 1,4

I.2.2

Khi cho mt axit vo nc th n s nhng proton cho nc v to ra ion H3O+

V d: HCl + H2O H3O+ + Cl-

S tch ly H+ ln hn so vi OH- lm cho dung dch c phn ng axit

Khi cho mt baz vo nc th n s thu proton cho nc v to ra 1 lng OH-

tng ng.

V d: NH3 + H2O NH4+ + OH-

S tch ly OH- ln hn so vi H+ lm cho dung dch c phn ng baz.

II. NH LUTIU KIN PROTON ( NH LUT BO TON

PROTON)

II.1 Ni dung nh lut proton (KP)

- Nu ta chn mt trng thi no ca dung dch lm chun (thng gi l

trng thi quy chiu hay l mc khng) th tng nng proton m cc cu t

mc khng gii phng ra bng tng nng proton m cc cu t thu vo

t ti trng thi cn bng .

Hay : mc khng : [H+] = ( [H+]i) cho - ( [H+]i)nhn

Trong mc khng c th l trng thi ban u, trng thi gii hn hay trng

thi ty chn

Vd1 : Vit biu thc KP i vi nc

Chn mc khng l H2O. ta c cn bng

H+ H2O OH

-

KP [H+] = [OH-]

Vd2 : Vit biu thc KP ca dung dch HCl

Mc khng l HCl, H2O

H+ H2O OH

-

HCl H+ Cl-

KP : [H+] = [OH-] + [Cl-]

Vd3: Trn 15,00 ml H2SO4 0,20 M vi 25,00 ml NaOH 0,04M. Xc nh thnh phn ca

hn hp v vit biu thc KP.

CH2SO4 = 0,075M ; CNaOH = 0,025 M

H2SO4 H+ + HSO4- NaOH Na+ + OH-

0,075 0,075 (M) 0,025 0,025 (M)

Phn ng: H+ + OH- H2O Kw-1 = 1014

0,075 0,025

0,050 _

Vy TPGH gm: H+ 0,050M ; HSO4- 0,075 M

Chn mc 0 l: H+, HSO4- v H2O.

KP : [H+] = CH+ + [OH-] + [SO42-]

III. CN BNG TRONG DUNG DCH N AXIT V N BAZ

III.1 Axit mnh

- khi nim: L cht phn li hon ton ra H+. K hiu l: HY

- Thng gp l cc axit ca Arenius.

- Cn bng:

H2O H+ + OH-

HY H+ + Y-

-KP:

[H+ ] = [OH- ] + [Y- ] = [OH- ] + CHY

TH1: [HY] 10-7 th thng b cn bng H2O.

TH2: [HY] 10-7 th khng b cn bng H2O. Tnh theo KP (nh lut tc dng

khi lng).

V d 1: Tnh pH, pOH v [H+ ], [OH- ] ca dung dch HCl 1,0.10-3M

HCl H+ + Cl-

1,0.10-3

H2O H+ + OH-

CHCl>> 10-7 vy [H+ ]=CHCl= 1,0.10-3 pH=-lg(1.10-3)=3

pOH=14-3=11 [OH- ] =1,0.10-11M

III.1.2 Baz mnh

- Khi nim: l cht phn li hon ton ra OH-

- Thng gp cc baz ca Arenius.

- Cn bng:

H2O H+ + OH-

XOH X+ OH-

- KP:

[H+ ] = [OH- ] - C

TH1: [OH-] 10-7 th b cn bng H2O.

TH2: [OH-] 10-7 th khng b cn bng H2O. Tnh theo KP (nh lut tc dng

khi lng)

Vd2: Tnh H+, OH-,pH cuar dung dch NaOH 1,0.10-4M

NaOH Na+ + OH-

1,0.10-4

H2O H+ + OH-

710NaOHC nn:

410.0,1 XOHCOH

10

4

14

10.0,110.1

10

OHK

H w

pH=-lg(1,0.10-10)=10,0

VD2: Tnh s gam NaOH phi cho vo hn hp thu c khi thm 8,00 ml HNO3

0,0100 M vo nc ri pha long thnh 500 ml pH ca dung dch thu c bng 7,50

(coi th tch khng thay i trong qu trnh ha tan).

Gi m l s gam ca NaOH cn tm: CNaOH = 103.m / 40.500 = m/20.

C HNO3 =

0,0100. 8,00500,00

= 1,60.10-4 M

V pH=7,5 (mi trng baz yu) d NaOH

Phn ng : HNO3 + NaOH NaCl + H2O

C0 1,6.10-4 m/20

C _ m/20 1,6.10-4

V pH=7,5 ~ 7,00 phi k n cn bng phn li ca nc.

Cc qu trnh xy ra trong dung dch:

NaOHd Na+ + OH-

H2O H+ + OH-

KP: [H+] = [OH-] - CNaOHd . Hay 10-7,5 = 10-6,5 (m/20 1,6.10-4)

m = 0,0032 (gam)

III.3 Dung dch n axit yu

- Khi nim:l l nhng axit phn li khng hon ton. K hiu l: HA.

- Thng gp: H2CO3, HAc, cation nh NH4+, anion nh HSO4-, dung dch trong nc

ca cation kim loi yu nh Cu2+,....

- Cn bng:

H2O H+ + OH-

HA H+ + A- ka

- KP:

[H+ ] = [OH- ] + [A-]

TH1: CHA 10-7 th b cn bng H2O. Tnh theo nh lut tc dng khi lng.

TH2: CHA -] 10-7 th tnh theo KP

KP:

w .[ ]K Ka HAhh h

h2 = Kw + Ka . [HA]

h= w .K C Ka gn ng

VD1: a) Cho bit in li ca dung dch axit HA 0,1M l 1,31%. Tnh pKa.

b) in li thay i th no nu pha long dung dch HA gp 10 ln ? Kt lun.

Gii: a) Chp nhn b qua s phn li ca nc

T nh ngha: [H+] = [A-] = C.HA = 0,1.1,31.10-2 = 1,31.10-3.

Xt cn bng: HA H+ + A- Ka

C 0,10

[ ] (0,10 1,31.10-3) 1,31.10-3 1,31.10-3

p dng LTDKL cho cn bng trn :

Ka =

H+ OH-

HA=

(1,31.10-3)2

0,1 - 1,31.10-3= 1,739.10-5 = 10-4,76

Kim tra gi thit gn ng: Ka.CHA = 10-4,76 >> Kw, do vic b qua s phn li ca

nc trn l chp nhn c. Vy: pKa= 4,76.

b) Pha long dung dch HA thnh 10 ln CHA = 0,01M

HA H+ + A- Ka = 10-4,76

[ ] 0,01-x x x

x2

0,01 - x= 10-4,76 x2 + 1,739.10-5x - 1,739.10-7 = 0 x = 4,08.10-4

= 4,08%

III.4 Dung dch n baz yu

- Thng gp: NH3, anion ca axit yu, cation CaOH+,

- Cn bng:

H2O H+ + OH kw

A- + H2O HA + OH- Kb =KwKa

- KP:

[H+ ] = [OH- ] - [HA]

TH1: Nu CA 10-7 th b cn bng H2O. Tnh theo nh lut tc dng khi lng.

TH2: CA 10-7 th tnh theo KP

h = Kwh

= Ka-1 [A-].h

h2 = Kw - Ka-1 [A-].h2

w11 .[ ]Kh

Ka A

- Gn ng: [A-] = CA-

- Gn ng lin tc:

III..5 Hn hp n axit v n baz

III.5.1 Hn hp axit mnh v axit yu

Gi s: C dung dch gm: axit mnh HY, C1 v axit yu HA, C2 H2O H+ + OH kw HY H+ + Y

HA H+ + A- ka

Hn hp axit nn phn li ca nc b qua.

HA H+ + A- ka

C C2 C1

[ ] (C2 x) (C1 + x) x

[ ].[ ][ ]a

H AKHA

=2

1.( 1)x CC x

TH1:

1

2

x C

x C

th 21

.aC

x kC

TH2: 12

x C

x C

th gii phng trnh bc 2

VD1: Tnh pH v cn bng trong dung dch thu c khi trn 50,00 ml NH3 2,00.10-3M

vi 50,00 ml H2SO4 2,00.10-3M.

Gii : CNH3 = 1,00.10-3M ; CH2SO4 = 1,00.10-3M.

Cc qu trnh : H2SO4 H+ + HSO4-

10-3 10-3

NH3 + H+ NH4+ Ka-1 = 109,24.

C0 10-3 10-3

C _ _ 10-3

TPGH: HSO4- 1,00.10-3M, NH4+ 1,00.10-3M

HSO4- H+ + SO42- Ka = 10-2

NH4+ H+ + NH3 Ka = 10-9,24.

H2O H+ + OH- Kw

So snh (1),(2) v (3) ta thy: Ka.CHSO4 >> Ka.CNH4+ >> Kw

cn bng (1) l ch yu: HSO4- H+ + SO42- Ka= 10-2.

C 10-3

[ ] 10-3 h h h

h2

10-3 - h= 10-2 h = 9,16.10-4 H+ SO42-= = h = 9,16.10-4 M

[HSO4-] = 8,40.10-5 M ; [OH-] = 1,09.10-11M

NH4+

NH3

= 10-39,16.10-4

9,16.10-4 + 10-9,24~ 1,00.10-3M

= 10-3. 10-9,24

9,16.10-4 + 10-9,24~ 6,28.10-10M

R rng [SO42-] >> [NH3] >> [OH-] nn vic tnh ton gn ng theo (1) l hon ton hp

l. Vy pH = 3,04.

III.5.2 Hn hp baz mnh v baz yu

Gi s: Dung dch gm: dunh dch baz mnh XOH, C1 v dung dch axit yu A-, C2.

H2O H+ + OH kw

XOH X+ OH-

C1 C1

A- + H2O HA + OH- Kb =KwKa

Hn hp baz nn b qua phn li ca nc. Tnh theo nh lut tc dng khi lng.

A- + H2O HA + OH-

C C2 C1

[ ] (C2 x) x (C1 + x)

Kb =2

1.( 1)x CC x

TH1: 12

x C

x C

th x= Kb. 21

CC

TH2: 12

x C

x C

th gii phng trnh bc 2.

III.5.3 Phn s nng

- ngha: nh gi quan h gia axit v baz.

- nh ngha: L t s gia nng cn bng ca cu t vi tng nng cc dng

tn ti ca cu t trong dung dch.

[ ] [ ] [ ],[ ].[ ]HA AHA A

HA HA AHA A C C

=>[HA]= .HA HAC

[A]= .A HAC

Theo nh lut bo ton nng , CHA =[HA] + [A-]

CHA =[HA] + [HA].Ka.h-1 = [HA].(1+ Ka.h-)

[HA]= CHA.a

hh K

CHA = .[ ] [ ]

A

h A AC

= [A-] .(h.Ka-1 +1)

[A-]= 1ah.K 1HAC

= CHA. aa

Kh K

III.5.4 Hn hp cc n axit

gi s trong dung dch c cc n axit : HA1, HA2, HA3. Ta c cc cn bng sau :

H 2 O

H A 1

H A 2

H A 3

H +

H +

H +

H

O H -

A 1

A 2

A 3

K w

K a 1

K a 2

K a 3

- Nu Ka1.C1 > > Ka2 >> Ka3 >> Kw th hn hp thnh n axit HA1 tnh

theo tc dng khi lng

- Nu Ka1.C1 Ka2 Ka3 Kw th tnh theo KP

KP : [H+] = [OH-] + [A1-] + [A-2] + [A-3]

hK w=h

K a1.[H A 1] K a2.[H A 2]K a3.[H A 3] h h h

Kw=h Ka1.[HA1] Ka2.[HA2] Ka3.[HA3] III.5.5 Hn hp cc n baz

- Hn hp gm : A1-, A2-, A3

Ta c

H 2 O

H A 1

H A 2

H A 3

H + O H -

A 1

A 2

A 3

K w

K a 1

K a 2

K a 3

+ H 2 O

+ H 2 O

+ H 2 O

O H -

O H -

O H -

K w .

K w

K w

-1

-1

-1

- Nu C1.Kb1>> C2Kb2 >> C3.Kb3 th tnh theo nh lut tc dng khi lng

- Nu C1.Kb1 C2Kb2 C3.Kb3 th tnh theo iu kin proton

KP : [H+] = [OH-] + [HA1] + [HA2] + [HA3]

hK w=h K a1.[A 1-1].h K a2.[A 2

-1].h K a3.[A 3-1].h-1-1-1

K w=h

K a 1 - 1 . [ A 1 - 1 ] K a 2- 1

. [ A 2 - 1 ] K a 3 - 1 . [ A 3 - 1 ]1 III.6.6 Hn hp axit yu v baz lin hp

Cho hn hp gm HA v A-. ta c :

H 2 O

H A 1

H + O H - K w

K a

O H -

H + A -

+A - + H 2 O H A K b

[ H + ] = K a [ H A ]

[ A -]= C a

C b

( 1 )

( 2 )

- Nu h > 10-7 , mi trng axit th tnh theo cn bng 1 (p dng LBTKL)

- Nu h < 10-7, mi trng baz th tin hs theo cn bng 2

- Nu h 10-7 th p dng KP

[H+] = [OH-] CA + [A-]

IV A AXIT V A BAZ IV.1 a axit (HnA) - Trong dung dch, a axit c kh nng phn li theo tng nc:

HnA H+ + Hn-1A- 1aK

Hn-1A- H+ + Hn-2A2- 2aK

....... ..... ........ ...... HA(n-1) H+ + An-

naK

C th coi a axit nh mt hn hp gm nhiu n axit. * Nu

naaaaKKKK .....

321 th c th coi s phn li ca a axit xy ra ch yu

nc 1 v c th tnh cn bng theo LTDKL p dng cho nc phn li th nht ca axit:

HnA H+ + Hn-1A- 1aK

[ ] C-x x x

xC

xK a

2

1 [H+] =x, t tnh c nng cc cu t khc

Ch : - Nhng axit c nhm OH nh trc tip vo nguyn t trung tm (H2CO3,

H3PO4...) th c pKa1 < pKa2. - Nhng axit c 2 proton nh trc tip vo nguyn t trung tm (H2S..) th pKa1 >Ka2>>Kw.nn b qua cn bng(2), (3), tnh ton da vo cn bng (1). H2S H+ + HS- Ka1=10-7 C0 0,1 [ ] 0,1-x x x

4101,0

10 42

7

pHHSHxx

x nn [OH-] = 10-10M.

Da vo cn bng (2) ta c: 92,122

292,12 10.10

S

HSSH M;

[H2S]=0,1-10-4 10-1M. V d 2: Dung dch axit H3PO4 c pH= 1,5. Xc nh nng mol/l ban u ca dung dch H3PO4. Bit H3PO4 c Ka1=10-2,15; Ka2=10-7,26;Ka3=10-12,32. Gii: Cc cn bng xy ra trong dung dch: H3PO4 H+ + H2PO4- Ka1 = 10-2,23 (1) H2PO4- H+ + HPO42- Ka2 = 10-7,26 (2) HPO42- H+ + PO43- Ka3 = 10-12,32 (3) H2O H+ + OH- Kw = 10-14 (4)

So snh: pH = 1,5 < pKa1 = 2,23; trong dung dch cn bng (1) xy ra l ch yu: H3PO4 H+ + H2PO4- Ka1 = 10-2,23 C0 x [ ] x-10-1,5 10-1,5 10-1,5

2014,010

)10(10 5,125,1

23,2

xx

.

Vy nng ban u ca H3PO4 l 0,2014 M. Dng 2: Hn hp a axit v axit mnh. V d 3: Tnh pH v nng cn bng ca cc cu t trong h gm HCl 0,010 M v H2S 0,10 M. Bit H2S c Ka1= 10-7; Ka2=10-12,9. Gii: V y l mi trng axit nn s phn li ca nc l khng ng k.. Cc cn bng xy ra trong h: HCl H+ + Cl- 0,010 0,010 H2S H+ + HS- Ka1 = 10-7 (1) HS- H+ + S2- Ka2 = 10-12,9 (2) V Ka1 >> Ka2 nn cn bng (1) l ch yu. H2S H+ + HS- Ka1 = 10-7 C0 0,10 0,010 [] 0,10-x 0,010+x x

.101,0

)01,0( 7x

xx Gi s x

Do Ka1>>Ka2>>Ka3 v Ka Ka1 v 2 axit cng nng nn phi tnh theo c cn bng (1) v (2). Nu chn mc khng l H3PO4 v HSO4- th KP l:

h

HSOKh

POHKSOPOHHh aa

4432442

..1

443.1 HSOKPOHKh aa (5) - Bc 1: v C= 0,010 Ka Ka1 Chn [H3PO4]0 = [HSO4-]0 = C/2=0,005 M v thay vo (5) tnh h1:

399,115,21 10.3,90050,0.100050,0.10 h

Thay gi tr h1 vo biu thc tnh [H3PO4] v [HSO4-] :

[H3PO4]1 = MhKhC

a

315,23

3

10.68,51010.3,9

10.3,9.01,0.1

[HSO4-]1 = MhKhC

a

399,13

3

10.67,41010.3,9

10.3,9.01,0.

- Bc 2: Thay gi tr [H3PO4]1v [HSO4-]1 vo (5) tnh h2:

13399,1315,2

2 10.4,910.76,4.1010.68,5.10 hh

Vy kt qu tnh lp gn ng. Vy pH = -lgh2 = 2,03. IV.2 a baz

- Trong dung dch, a baz c kh nng thu proton tng nc ca nc. Qu trnh proton ho ca a baz xy ra ngc vi qu trnh phn li ca a axit tng ng:

An- + H2O HA(n-1)- + OH- 1.1

nawbKKK

HAn-1 + H2O H2A(n-2)- + OH- 1 12 .

nawbKKK

..... ..... ..... ..... Hn-1A- + H2O HnA + OH- 11.

awb KKK n

*Nu nbbbb

KKKK .....321

, ngha l qu trnh proton ho nc 1 chim u th,

khi c th nh gi thnh phn cn bng ca a baz nh mt n baz : An- + H2O HA(n-1)- + OH- 1bK

[ ] C-x x x

Ta c : xC

xKb

2

1 x=[OH-], t nh gi c thnh phn cn bng ca h.

* Lu : i vi cc a axit v a baz c cc hng s phn li k tip gn nhau th phi p dng KP tnh lp nng [H+] cho n hi t. Dng 1: Dung dch cha mt a baz V d 1: Tnh pH ca dung dch Na2S 0,010 M. Bit H2S c Ka1= 10-7; Ka2=10-12,92. Cc qu trnh xy ra trong dung dch: Na2S 2Na+ + S2- S2- + H2O HS- + OH- Kb1 = 10-14/10-12,92= 10-1,08 (1) HS- + H2O H2S + OH- Kb2 = 10-14/10-7= 10-7 (2) H2O H+ + OH- Kw= 10-14 (3) V Kb1>>Kb2>>Kw nn trong dung dch cn bng (1) xy ra l ch yu. S2- + H2O HS- + OH- Kb1= 10-1,08 C0 10-2 [] 10-2-x x x

p dng LTDKL ta c: 0101010

10 08,308,1222

08,1

xx

xx

x= 9.10-3 = [HS-] = [OH-] [H+] = 1,1.10-12 pH = 11,95. V d 2: Tnh khi lng mui Na2S phi cho vo 1 lt nc c dung dch c pH = 11,5. Bit H2S c Ka1= 10-7; Ka2=10-12,92. Gii : Cc cn bng xy ra trong dung dch l:

Na2S 2Na+ + S2- a a S2- + H2O HS- + OH- Kb1 = 10-14/10-12,92= 10-1,08 (1)

HS- + H2O H2S + OH- Kb2 = 10-14/10-7= 10-7 (2) H2O H+ + OH- Kw= 10-14 (3) V Kb1>>Kb2>>Kw nn trong dung dch cn bng (1) xy ra l ch yu. S2- + H2O HS- + OH- Kb1= 10-1,08 C0 a [ ] a-x x x

xax

2

08,110 trong x= [OH-] =10-14/ 10-11,5 = 10-2,5

a = 3,28.10-3 = SNaC 2 gm SNa 256,078.10.28,33

2

Dng 2: Hn hp a baz v baz mnh

V d 3: Trn 10,00 ml dung dch NaOH 8,00.10-3 M vi 30,00 ml dung dch H2S 1,00.10-3 M. Tnh pH ca dung dch thu c. Bit H2S c Ka1= 10-7; Ka2=10-12,92. Gii: y l bi ton pha trn gia a axit (H2S) vi baz mnh nn c phn ng xy ra, chng ta cn xc nh thnh phn gii hn, t m t cc cn bng xy ra trong dung dch v tnh pH.

- Nng ban u ca cc cht trong dung dch: MCNaOH 33

10.240

10.10.8

MC SH3

3

10.75,040

30.10.12

NaOH + H2S NaHS + H2O 0,75.10-3 0,75.10-3 0,75.10-3 NaOH + NaHS Na2S + H2O 0,75.10-3 0,75.10-3 0,75.10-3 TPGH gm: Na2S 0,75.10-3 M; NaOH d 0,5.10-3 M. Vy dung dch l hn hp a baz ( S2-) v baz mnh, nn ta c cc cn bng xy ra: S2- + H2O HS- + OH- Kb1= 10-1,08 (1) HS- + H2O H2S + OH- Kb2= 10-7 (2) H2O H+ + OH- Kw= 10-14 (3) V Kb1>>Kb2>>Kw nn trong dung dch cn bng (1) xy ra l ch yu. S2- + H2O HS- + OH- Kb1= 10-1,08 C0 0,75.10-3 0,5.10-3 [ ] 0,75.10-3-x x 0,5.10-3+x

p dng LTDKL ta c: xxx

3

308,1

10.75,0)10.5,0(10

Gii PT bc 2 ta c: x = 7,4.10-4 [OH-]=1,24.10-3 [H+]=8,06.10-12 pH= 11,09.

V.CC CHT IN LY LNG TNH - Cc cht va c tnh axit (cho proton) va c tnh baz (thu proton) c gi l

cc cht lng tnh. - Cc cht lng tnh gm:

+ Cc hidroxit ca 1 s kim loi (nhm, km, crom, thic, ch) + Cc aminoaxit l nhng cht in li lng tnh do s c mt ng thi trong phn t cc nhm amin c tnh baz v nhm cacboxyl c tnh axit. (axit -aminopropionic C3H7O2N...) + Cc mui axit l cc mui c to thnh do s trung ha khng hon ton cc a axit. (NaH2PO4, Na2HPO4...) + Mui ca axit yu v baz yu v ion ca chng cng c kh nng cho v nhn proton (CH3COONH4, NH4HCO3...)

- Xt dung dch mui NaHA, trong dung dch c cc cn bng NaHA Na+ + HA-. H2O H+ + OH- Kw (*) HA- H+ + A2- Ka2 (**) HA- + H+ H2A Ka1-1 (***)

Nu chn mc khng l : HA-; H2O. Ta c biu thc KP: h= [H+] = [OH-] + [A2-] - [H2A] (4*) Sau khi t hp cn thit ta c:

h = ].[1

].[1

1

2

HAK

HAKK

a

aw (5*)

Trong a s trung hp HA- phn li yu nn c th coi gn ng [HA-] CNaHA = C, v vy biu thc (5*) tr thnh :

h = CK

CKK

a

aw

.1

.1

1

2

(6*)

- Nu Kw > 1 hay Ka1 Ka2 nn dung dch c phn ng baz.

Kim tra cc iu kin gn ng ta c: Kw

s b HA trung ho to ra mt lng tng ng baz yu A-, lng ny tng i b so vi lng baz A- c sn, v vy pH tng ln khng ng k.

V d 1: nh gi s thay i pH ca hn hp m A gm c CH3COOH 0,1M v

CH3COONa 0,1M

a) Khi thm 0,0010 mol HCl vo 1,000 lt dung dch A

b) Khi thm 0,0010 mol NaOH vo 1,000 lt dung dch A

Bi lm:

Trong dung dch A c cc cn bng:

CH3COONa Na+ + CH3COO-

H2O H+ + OH- Kw (1)

CH3COOH H+ + CH3COO- Ka (2)

V Ka.Ca / Cb= 10-4,76 >>10-7 nn c th coi qu trnh (2) l ch yu v c th p dng s

Kamar tnh cn bng.

CH3COOH H+ + CH3COO- Ka= 10-4,76

C 0,1 0,1

C -x x

[ ] 0,1-x x 0,1+x

Vi x

p dng nh lut tc dng khi lng ta tnh c :

76,410101,0

)099,0(

aKxxx

x= [H+] = 1,77.10-5M, pH= 4,75

b) Khi thm 1.10-3 mol NaOH

Phn ng CH3COOH + NaOH CH3COONa + H2O

Co 0,1 0,001 0,1

C 0,1- 0,001 - 0,101

TPGH: CH3COOH 0,099M ; CH3COONa 0,101M

Cn bng: CH3COOH H+ + CH3COO-

C 0,099 0,101

[ ] 0,099-x x 0,101+x

[x(0,101+x)]/(0,099-x)= Ka = 10-4,76

x

-Khi nim: L s mol bazo mnh (hoc axit mnh) cn cho vo 1 lt dung dch

m lm tng (hoc gim) pH 1 n v.

= dpHda

dpHdb

l m nng, db l s mol bazo mnh, da l s mol axit mnh thm vo lm

bin i dpH n v pH, du (-) ch pH gim khi thm axit.

Nhn xt: Khi cng ln th h c kh nng m cng ln. Gi tr m nng ph

thuc vo nng cc axit v bazo lin hp dng pha ch dung dch m, v ph

thuc t l nng gia chng.

i vi dung dch m gm n axit yu v bazo lin hp c th tnh theo biu

thc tng qut:ng nhau v c gi tr bng:

=2,3.

HHK

HKHCK w

a

a2)(

Khi [H+], [OH-] 0, cn khi gim th pH1/2 < 0.

Bng: pha long pH1/2 (theo n v pH) i vi hai loi dung dch m pH

= 4,5 9,5 cc nng C khc nhau.

H m C mol/l

HA + A- HA+ + A

0,1 0,028 -0,028 0,05 0,025 -0,025 0,025 0,019 -0,019 0,02 0,017 -0,017 0,01 0,013 -0,013 0,005 0,010 -0,010

0,0025 0,007 -0,007

VI.4 ng dng ca cc dung dch m Cc dung dch m pH c ng dng rng ri trong thc t phn tch.

- Phn ng tch: nhiu phn ng phi c thc hin pH xc nh, c thit lp trog mi trng m thch hp. Vd: tch Ba2+ ra khi Sr2+ di dng kt ta BaCrO4 cn duy tr pH trong khu vc 4 5 bng hn hp m axetat (CH3COOH + CH3COO-)...

- Phn ng sinh ha: ph thuc rt mnh vo pH + Cc h m c pH xc nh ny phi tr v mt ha hc. + Cc cu t ca h m phi tan tt trong nc v khng c thm thu qua cc mng t bo. + Cc h m c dng kim tra pH trong khu vc 6 8. Thng dng cc axit v baz hu c phc tp. (tham kho bng 2.2 gio trnh Ha hc phn tch 1 cn bng trong dung dch / trang 90) - pha ch dung dch m c = max ng vi pH = pKa cn kim ha hoc axit

ha cc cht cho theo t l thch hp.

- Mt s h m c khu vc pH rng hn (t 3-8) c iu ch t cc dn xut

ca

2,6- imetylpiridin

VII.CN BNG TO PHC HIROXO TRONG DUNG DCH NC CA CC ION KIM LOI.

VII.1 Khi nim Trong dung dch nc cc ion kim loi tn ti di dng hidrat ha. Chng l

nhng axit c kh nng cho proton v to ra cc phc hiroxo kim loi. c tnh axit ca

cc ion kim loi ph thuc vo bn cht ca ion kim loi, vo in tch ca chng.

Cc ion kim loi kim khng th hin tnh axit v hidroxit ca chng l nhng

bazo rt mnh. Cc ion kim loi kim th l nhng axit rt yu. Cc ion trng thi oxi

ha cao thng c tnh axit hn cc ion cng loi trng thi oxi ha thp hn.

C th m t s to phc hidroxo ca ion Mn+ theo s tng qut:

Mn+.mH2O [MOH.(m-2) H2O](n-1)+ + H3O+ *1

Mn+.mH2O [M(OH)2.(m-4) H2O](n-2)+ + 2H3O+ *2

Hoc vit di dng n gin hn:

Mn+ + H2O MOH(n-1)+ + H+ *1

Mn+ + 2H2O M(OH)2(n-2)+ + 2H+ *2

y, *1 = Kw1 ; *2= Kw2:

1=

OHMMOHn

2 =

22)( OHM

OHMn

V d:trong nc ion Cu2+ to thnh cc phc hidroxo nh sau:

Cu2+ + H2O CuOH+ + H+ lg*1= -8,0

Cu2+ + 2H2O Cu(OH)2 + 2H+ lg*2= -6,8

Cu2+ + 3H2O Cu(OH)3- + 3H+ lg*3= -27,2

Cu2+ + 4H2O Cu(OH)4- + 4H+ lg*4= -40,3

-Khi kim ha dung dch cha cc ion kim loi th xut hin cc phc hidroxo bc cao

cng vi kt ta hidroxit kim loi.

-Nhiu ion kim loi a ha tr to c cc phc hidroxo nhiu nhn:

V d:

2Fe2+ + 2H2O Fe2(OH)24+ + 2H+ lg*22= -2,85

2Al3+ + 2H2O Al2(OH)24+ + 2H+ lg*22= -14,56

- Vic nh gi thnh phn cn bng to phc hidroxo ni chung l kh phc tp. Trong

trng hp n gin khi dung dch c mi trng axit r v c th b qua s to thnh

ca cc phc hidroxo bc cao th c th tnh gn ng cn bng theo LTDKL nh i

vi cc h n axit thng thng

p dng:

Trong trng hp n gin, khi dung dch c mi trng axit r v c th b qua s to thnh cc phc hidroxo bc cao th c th tnh gn ng cn bng theo nh lut tc dng khi lng nh i vi cc h n axit thng thng.

VIII CC CHT CH TH AXIT-BAZ - Cc cht ch th axit - baz c s dng nh gi gn ng pH ca cc dung

dch. - Yu cu c bn: phi thay i mu thun nghch vi pH ca dung dch.

Mun vy, cht ch th phi l mt axit hoc mt baz,m mu ca ca dng axit v dng baz lin hp phi khc nhau.

- Cc cht ch th thng l nhng axit hoc baz hu c. - Cht ch th axit-baz c tng hp u tin l phenolphtalein.

VIII.1 Khong pH chuyn mu ca cht ch th - Mi cht ch th thay i mu mt khu vc pH xc nh m ta gi l khong pH

chuyn mu. - Ta k hiu dng axit l HIn v dng baz lin hp l In-. Cn bng phn li ca cht

ch th c vit: HIn H+ + In-

Theo LTDKL, ta c hot ion hidro:

(H+) = Ka. . V

pH= -lg(H+) = pKa + HIn

In-

+ lg

fIN-

fHIN (1) - Trong trng hp c th coi fIn- = fHIn = 1 th biu thc (1) tr thnh

pH = pKa + lgHIn

In-

(2)

- Mu ca cht ch ph thuc t s HIn

In-

m t s ny theo (2) li ph thuc pH. pH cao th [In- ] >> [HIn], cht chi th tn ti ch yu dng baz pH thp th [Hin] >> [In-], dng axit chim u th.

- Thng thng mt ta khng th phn bit s tn ti ca c 2 dng c mu nu nng ca mt trong hai dng vt qu 10 ln nng ca dng kia.

+ Nu [HIn] > 10[In-] th mt ta ch nhn thy mu ca dng HIn. y pH < pKa 1.

+ Nu [In-] > 10[HIn] th mt ta ch nhn thy dng In: y pH > pKa + 1 Nh vy cht ch th ch thay i mu trong khong:

pKa -1 < pH < pKa + 1 - Khong pH ny c gi l khong pH chuyn mu ca cht ch th.

V d: khong chuyn mu ca qu l 5 < pH < 8 c ngha l: + pH < 5 qu tn ti dng Hin (mu ) + pH > 8 dng In- (xanh) + T 6 n 8 cht ch th c mu t tm sang xanh tm. Trong bng 2.3 (gio trnh trang 96) c gii thiu cc cht ch th ph bin, khong chuyn mu v cc dng axit, baz ca chng.

Bng 2.3. c trng mt s cht ch th axit baz Mu Cht ch th Khi lng

mol (M) pKa Khong pH

chuyn mu Dng axit (HIn)

Dng baz (In)

Thymol xanh Metyl da cam Bromphenol xanh Metyl Bromcrezol ta Qu trung tnh Bromthymol xanh Phenol o Crezol - Naphtolphtalein Phenolphtalein Thymol xanh

466,27 327,2 669,8 269,14 540,02 215,21 288,65 642,09

322 386,5 422,26 318,11 466,27

1,65 3,52 4,10 5,0 5,40 6,6 6,85 7,3 8,08 8,46 8,4 9,77 9,20

1,2 2,8 3,1 4,4 3,0 4,6 4,2 6,3 5,2 6,8 5,0 8,0 6,8 8,0 6,0 7,6 6,4 8,2 7,2 8,8 7,0 9,0

8,2 10,0 8,0 9,6

Vng

Vng Vng Vng Vng

Khng mu Vng

Vng Vng Ta

Vng Ta

Xanh Da cam Xanh Ta Lc

Xanh

Thymolphtalein 741,23 9,7 9,3 10,5 Khng mu Xanh

VIII.2 Cc cht ch th hn hp

t vn : Cc cht ch th n ch cho php nh gi gn ng pH trong mt khu vc pH xc nh vi chnh xc 1 n v. Chng hn,dng qu tm ch c th phn bit cc dung dch c pH t 5 - 8. Khng th phn bit cc dung dch c pH < 5 v trong cc dung dch ny qu u c mu . Cng vy cc dung dch c pH > 8 th qu u c mu xanh. Cc dung dch c pH t 5 - 8 c mu t tm sang tm xanh. y ch c th phn bit cc dung dch c mu tm (pH ~ 5), tm (pH ~ 7) v tm xanh (pH ~ 8). Gii quyt vn : xc nh pH chnh xc hn v m rng khu vc xc nh pH, ngi ta dng cc cht ch th hn hp sao cho s i mu xy ra mt khong pH tng i hp v c s thay i mu lin tc theo s thay i pH. Ngi ta thng trn cc cht ch th khc nhau vi t l thch hp sao cho khi pH tng th mu bin i dn t sang tm. Trong bng 2.4 (gio trnh trang 97) c cho thnh phn ca 3 loi ch th hn hp (vi nng g/l trong dung dch ru etylic 60%).

Bng 2.4. Thnh phn ca mt s loi ch th hn hp

Cht ch th thnh phn

Ch th hn hp N01

Ch th hn hp N02

Ch th hn hp N03

Metyl da cam Metyl vng Metyl Bromthymol xanh Naphtolphtalein Crezolphtalein Phenolphtalein Alizarin vng G

0,03 -

0,15 0,30

- -

0,35 -

- 0,02 0,03 0,40 0,40 0,20 0,20

-

- 0,01

0,013 0,16 0,24 0,16 0,40 0,30

Sau y l s bin i mu sc theo pH ca cht ch th hn hp N01: pH 3 4 5 6 7 8 9 10

Mu da cam Da cam Vng Vng luc Xanh luc Xanh Tm

IX PHN NG AXIT- BAZ TRONG DUNG MI KHC NC

IX.1 Phn loi cc dung mi *V nguyn tc,mi cht lng u c th dung lm dung mi.

- Cc dung mi phn t trong tn ti ch yu cc phn t khng mang in (nc, ru, ammoniac lng, cc axit cacboxylic, cc amin).

- Cc dung mi ion trong ch c cc cation v anion ( nhit cao) v loi dung mi kim loi v kim loi nng chy.

Cc dung mi phn t ng vai tr quan trng, trong cc dung mi lng tnh l ph bin hn c.

IX.2 Phn ng axit baz trong dung mi lng tnh - Trong cc dung mi lng tnh c qu trnh t phn proton ca cc phn t dung

mi. Chng hn, vi dung mi HS HS p+ + S- HS + p+ H2S+ 2HS H2S+ + S-

- Tch s ion ca dung mi [H2S+] [S-] = Ki V d: phn ng t nhn proton ca mt s dung mi lng tnh: Nc: 2H2O H3O+ + OH- Ki = 1,00.10-14 Ru etilic: 2C2H5OH C2H5OH2+ + C2H5O- Ki = 1,00.10-19

Axit fomic: 2HCOOH HCOOH2+ + HCOO- Ki = 1,00.10-6 Cng nh nc, trong dung mi HS ta c pH = -lg[H2S+]; pS = - lg[S-] V pKi = pH + pS

Trong dung mi nguyn cht pH = pS =

12 .pKi

Dung dch s c phn ng axit khi [H2S+] > [S-] hoc pH < pS, s c phn ng baz khi [H2S+] < [S-] hoc pH > pS. Thang pH c xc nh trong phm vi pH = 0 cho n pH = pKiHS.

Trong bng 2.5 (gio trnh trang 99) c cho c tnh axit baz ca mt s dung mi lng tnh.

Bng 2.5. c tnh ca axit baz ca mt s dung mi lng tnh

Dung mi (HS)

Ion lioni Ion liat Ki Thang pH pH trung tnh

H2SO4 H3SO4+ HSO4- 10-3,8 0 3,8 1,9 HCOOH HCOOH2+ HCOO- 10-6,0 0 6,0 3,0 CH3COOH CH3COOH2+ CH3COO- 10-14,4 0 14,4 7,2 CH3OH CH3OH2+ CH3O- 10-16,7 0 16,7 8,35

C2H5OH C2H5OH2+ C2H5O- 10-19,0 0 19,0 9,5 NH3 (lng) NH4+ NH 10-22,0 0 22,0 11,0

Khi cho axit HA vo trong dung mi HS th xy ra cc qu trnh: HA p+ + A- KA p+ + HS H2S+ K-1H2S+ HA + HS H2S+ + A- Ka = KA . K-1H2S+ (*) - Tnh axit ca HA ph thuc vo: + Tnh axit ca cht in ly tan (KA) + Tnh baz ca dung mi (thu proton ca dung mi). VD: Trong nc axit axetic l mt axit yu nhng trong dung mi NH3 lng th do tnh baz ca dung mi tng ln nn axit axetic li l mt axit mnh: CH3COOH + NH3 CH3COO- + NH4+ Trong nc, HCl l mt axit mnh, nhng trong dung dch axit axetic bng c tnh baz rt yu nn HCl li l mt axit yu: HCl + CH3COOH CH3COOH2+ + Cl-

Khi ha tan baz B vo dung mi HS s c cc qu trnh: HS p+ + S- KHS B + p+ BH+ K-1BH+

B + HS BH+ + S- KbBv = KHS K-1BH+ (**) - Tnh baz ca B ph thuc: + Tnh baz ca cht in ly tan (thu proton ca B) + Tnh axit ca dung mi HS. -> Dung mi cng c tnh axit th tnh baz ca cht in li cng mnh. - i vi cc dung mi c hng s in mi thp th thng xy ra s lin hp gia cc ion tch in ngc du. VD: Trong dung dch axit HA cn xy ra s lin hp gia cc ion H2S+ v A- to ra cp ion H2S+A- (hay H+A-) khc vi phn t HA. Do trong cc dung mi ny, lc axit ca nhiu cht gim i. trong nc ( = 78,5), axit axetic c pKa = 4,76, cn trong ru etylic ( = 24,2) th pKa ca axit axetic bng 10,3, ngha l lc axit gim i nhiu.

- Do khng phn bit c cp ion vi phn t khng phn li ca axit nn ngi ta biu din chung hng s phn li ca axit HA di dng:

Ka = = Ngha l coi tng nng dng phn t v nng cp ion bng nng axit khng phn li.

- Hng s phn li ca axit Ka ph thuc vo hng s in mi ca dung mi, ph thuc in tch v kch thc cc ion.

T (**) ta c hng s baz: Kb = KHSKa-1

tng t nh nc. - Trong bng 2.6 (gio trnh trang 101) cung cp ch s hng s axit v ch s hng

s baz ca mt s cp axit baz trong etanol v trong axit axetic bng. Bng 2.6. Ch s hng s phn li axit pKa v ch s hng s phn li baz pKb ca

mt s cp axit baz trong cc dung mi etanol v CH3COOH bng.

Cp axit baz Dung mi pKa pKb HNO3/NO3-

HCOOH/HCOO- CH3COOH/CH3COO- HClO4/ClO4 HCl/Cl-

HNO3/NO3

C2H5OH C2H5OH C2H5OH CH3COOH bng CH3COOH bng CH3COOH bng

3,6 9,2 10,3 2,9 5,0 9,4

15,4 9,8 8,7 11,5 9,4 5,0